Stainless steel alloys containing 13% chromium are known. An article by Georg Fischer-Aktiengesellschaft, Schaffhausen (Schweiz) and prepared for publication in the Revue de la Metallurgie, July/August 1966 relates to a high strength 13% chromium cast steel of improved weldability. The author discusses the modification of the classical 13% chromium steel to improve its weldability. His alloy composition contains
C 0.04-0.06% PA1 Cr 12-13% PA1 Ni 3.5-3.9% PA1 Mo 0.5% PA1 C (max.) 0.06% PA1 Cr 12.5% PA1 Ni 3.8% PA1 Mo 0.5% PA1 C 0.08% max. PA1 Mn 1.50% max. PA1 Si 1.50% max. PA1 Cr 11-14% PA1 Ni 3.0-4.5% PA1 Mo 1.00% max. PA1 Fe -- Balance PA1 Carbon: 0.05-0.07 PA1 Manganese: 0.70-1.00 PA1 Silicon: 0.30-0.50 PA1 Phosphorus max.: 0.020 PA1 Sulfur-max.: 0.020 PA1 Nickel: 3.50-4.25 PA1 Chromium: 11.20-12.25 PA1 Molybdenum: 0.30-0.50 PA1 Tin: 0.03 max. PA1 Aluminum: 0.03 max. PA1 Vanadium: 0.03 max. PA1 Iron: Balance
The author concludes that the following cast steel composition presents undeniable advantages in comparison with the classical 13% chromium steel:
Another article is that of the Esco Corp. of Portland, Oreg. The article is entitled Alloy Notebook No. 13 and contains
However, the specific alloy that Esco apparently uses contains 13% chromium and 4% nickel and is known as Alloy 13-4.
Reference is also made of U.S. Pat. Nos. 3,378,367 -- Lars Eije Friis et al. and 3,385,740 -- Baggstrom et al. These patents disclose steel alloys containing 11-14% chromium and 4-8% nickel. U.S. Pat. No. 3,378,367, however, relates to steel which is martensitic in structure but contains dispersed austenite. U.S. Pat. No. 3,385,740 discloses an austenitic-martensitic steel.
Another alloy is known which contains 11.25-13% chromium, 0.06 to 0.15% C and .20% molybdenum. However, the alloy contains only 0.50% nickel (max.). Such alloys are essentially the same as AISI 410 stainless steel.
The prior art states that the content of chromium can be lowered somewhat, see the article by Georg Fischer, if the carbon content is low. If silicon is present it must be limited to prevent the formation of ferrite. If the nickel content is increased a martensitic microstructure is obtained.
Unexpectedly, applicants have discovered an alloy, which falls within an area of the prior art, but which gives better high strength and high toughness than those of the prior art, and which is highly adaptable for making stainless steel forgings and turbine buckets.